Make normalizeSlashes a no-op there are no bad slashes

On Windows, there will probably be a negligible slowdown, iterating over
the pre-slash prefix of each unnormalized path (though we might come out
ahead if paths are normalized more than once).

On *nix, this saves work - 1.8s -> 0.4s in the project I'm
investigating.

Author: amcasey

src/compiler/path.ts

@@ -452,7 +452,9 @@ namespace ts {
      * Normalize path separators, converting `\` into `/`.
      */
     export function normalizeSlashes(path: string): string {
-        return path.replace(backslashRegExp, directorySeparator);
+        return path.indexOf(altDirectorySeparator) === -1
+            ? path
+            : path.replace(backslashRegExp, directorySeparator);
     }
 
     /**